Bearing materials with plastic-based overlays are known as single-layer, two-layer or three-layer composite materials: solid plastic bearings, bearings with an outer metallic backing and directly applied or adhered plastics, bearings with inner wire meshes, as well as three-layer bearings of backing metal, a sintered porous metal layer and a covering layer formed in the pores. All these bearings are generally used in fields in which the use of externally supplied lubricants is impossible or undesirable. When in operation, therefore, they must intrinsically provide the lubricants.
Multilayer materials differ from solid plastic materials, for example by a negligible tendency towards cold flow under load, by substantially better heat conductivity and, in connection therewith, by notably higher possible pv values. The solid plastic materials may, however, be advantageous in certain cases, e.e. for reasons of cost. Among three-layer materials, it is possible to distinguish further between those with overlays based on fluorothermoplastics, such as PTFE, PFA, FEP etc., and other plastics such as PEEK. The latter two groups differ in their manner of operation: while, in the case of PTFE-based materials, the bronze intermediate layer is the "active" component of the overlay and acts like a filler, the other plastic materials use it only as an anchoring means. If there is sufficient affinity to the metal backing, they permit the production of true two-layer materials, but they may also be applied with the aid of an adhesive layer. On the active sliding surface the thermoset or thermoplastic material assumes the supporting role of the bronze. Moreover, bearing materials of filled fluorothermoplastic films adhered to metal or other supports with wire meshes incorporated in the plastic are known and may likewise be adhered to a metal backing.
Each variant has inherent advantages and disadvantages. However, the three-layer materials based on fluorothermoplastics, such as PTFE, are universally applicable and easy to produce and also exhibit the highest performance and temperature resistance. In the case of the latter, homogeneous mixtures are produced by means of a plastic dispersion and the final composite material is produced, after the production of a PTFE/filler paste, by a rolling process and subsequent sintering of the PTFE.
The most commonly used fillers for such materials are lead and molybdenum sulphide, which produce almost identical performance and which may be also used with lubrication. In many cases it would be desirable to solve structural problems by using maintenance-free, space-saving plain bearings with a PTFE overlay, but this must be rejected owing to the upper load limit thereof, which, within an average load and speed range (0.5-100 MPa and 0.02-2 m/s), lies at a pv value of approximately 2 MPa m/s.
A sliding material is known from DE 41 05 657 A1 which comprises a backing metal with a porous metal layer, in which definite pores are present, the pores and their surfaces being impregnated and coated with an impregnating coating composition. The impregnating coating composition consists of 0.5-30 vol.-% PFA, EPE or FEP, 5-30 vol.-% metallic lead with a specific surface of from 1000-8500 cm.sup.2 /g in an average particle size, 0.5-30 vol.-% metal oxide, metal fluoride, graphite or the like, ceramics, such as SiC, and fibrous material, such as carbon fibers or glass fibers, the rest consisting of PTFE. The total of all components other than PTFE amounts to from 6-50 vol.-%.
DE 41 05 657 A1 mentions only a metal fluoride-containing impregnating material comprising 2% PbF.sub.2 but no metal oxide, wherein said material is not distinguished from other exemplary embodiments comprising no metal fluoride by improved cavitation resistance or wear resistance.
EP 0 183 375 A2 describes a bearing especially for shock absorbers, whose overlay consists, for the purpose of improving the cavitation resistance, of PTFE with a fluoride of low water-solubility. The fluoride content, which may, inter alia, be CaF.sub.2 or MgF.sub.2, is stated as being from 1-50 vol.-%, preferably from 10-30 vol. %. 20 vol. % is cited as a preferred example. GB 912,793 describes standard bearing materials of PTFE with lead and/or lead oxide, wherein the filler content is from 16-44%, preferably from 16-30%.
The self-lubricating bearing known from DE 35 16 649 A1 comprises thinly rolled-out sheet- or scale-like particles in the PTFE matrix, which form smears which consist of a plurality of layers arranged spacedly one above the other and extending substantially parallel to the surface of the sintered metal layer. Metal oxides and metal fluorides, inter alia, are cited as solid lubricants, only lead and lead oxide being discussed as Examples.